The present invention relates to tellurite glass which is doped with rare earth ions, and is suitable for the fabrication of optical waveguides.
The demand for telecommunications transmission capacity continues to increase as more data, voice, and video signals are transmitted through the Internet and because of emerging multimedia applications. The demand is addressed by the recent commercial availability of optical fibers (the xe2x80x9cAllWave(trademark)xe2x80x9d fiber) having lower loss than conventional silica fibers in the wavelength region 1280 to 1700 nm. To exploit this advantageous loss characteristic, there is currently a strong interest in the development of optical amplifiers designed to cover a larger or different bandwidth than known amplifiers such as the widely used erbium doped fiber amplifier (EDFA). The EDFA typically operates in the so-called C (conventional) amplifier band, covering 1530 to 1565 nm.
The so-called S (short) band, covering wavelengths in the range 1460 to 1530 nm, can be accessed by the use of thulium ions (Tm3+) as a dopant in a glass host material. The energy level structure of Tm3+ ions permits radiative emission around 1470 nm. Desirable characteristics in an optical gain medium, for use in optical amplifiers and oscillators, include high gain and high gain flatness over the entire spectral range of interest. To achieve these for the Tm3+ ions, it is important to choose a suitable host glass matrix. Also, the glass should be suitable for the manufacture of optical fibers, if fiber amplifiers are to be successfully fabricated.
Silica is widely used as a glass host material for optical fibers and other waveguide structures. However, it is not suitable for the 1470 nm emission from Tm3+ ions, because it has too large a phonon energy. The 1470 nm emission originates from the Tm3+ 3H4 energy level, which is spaced apart from the lower 3H5 level by an energy gap of xcx9c4400 cmxe2x88x921 when the ions are doped into glasses. To reduce undesirable non-radiative decay from the 3H4 level, the emission of more than five phonons is required to bridge this energy gap. Silica has a phonon energy of xcx9c1100 cmxe2x88x921, so the number of phonons which is equal to the ratio of the xcx9c4400 cmxe2x88x921 energy gap for the 1470 nm transition in Tm3+ is only four. Hence the 3H4 level decays predominantly non-radiatively, so that the 1470 nm Tm3+ transition is not radiatively efficient in a silica host.
It is therefore necessary to look for a glass host with a lower phonon energy. Fluoride glasses are a possibility. For example, the phonon energy of a zirconium fluoride-based glass is only 550 cmxe2x88x921, and the Tm3+ 3H4 transition is 100% radiative. Fiber amplifiers and lasers based on fluoride glass doped with thulium and operating at 1.47 xcexcm have been demonstrated by Aozasa et al [1] and Komukai et al [2]. However, fluoride glasses are disadvantageous owing to poor glass stability and chemical durability, and their hygroscopic nature. Similar objections apply to phosphate and borate glasses.
Tellurite glasses, which are a large family of glasses containing tellerium oxide, TeO2, have also been used to host rare earth dopants. Many compositions of tellurite glass have been made. For example, U.S. Pat. No. 3,855,545 [3] reports neodymium doping in a tellurite glass of the form TeO2:BaO:Li2O which was used to make laser rods. Wang et al [4] used the similar composition TeO2:NaO2:ZnO (with and without Bi2O3) to fabricate a single mode fiber laser. This composition has also been reported in U.S. Pat. No. 5,251,062 [5], which is directed primarily to doping with erbium, but suggests doping with thulium for operation around 2 xcexcm. Erbium doping in the same composition is also reported by Choi et al [6], who looked at energy transfer between erbium and cerium ions. Further studies of this composition include: erbium doping to produce an EDFA [7]; doping with praseodymium to make 1.3 xcexcm optical amplifiers [8]; praseodymium-ytterbium co-doping, again directed to 1.3 xcexcm operation and to study energy transfer between the codopants [9]; doping with neodymium and praseodymium, together with substitution of the sodium for other alkalis [10]; and thulium-dysprosium co-doping, to look at the effect of the dysprosium on the thulium emission spectra [11]. EP 0 858 976 [12] describes a number of tellurite glasses all containing Bi2O3 and doped with various rare earth metals. Jiang et al [13] have considered tellurite glasses containing La2O3 which were doped with erbium to achieve a laser material with a high emission cross-section. Neindre et al [14] studied the effects of alkali content on absorption linewidth in erbium-doped tellurites containing oxides of two different alkalis. The use of tellurite glass for frequency conversion has been reported by Tanabe et al [15], who studied the composition TeO2:BaO:ZnO co-doped with varying levels of thulium and erbium.
As is apparent from the preceding paragraph, the tellurite glasses have been studied in some detail. However, many of the compositions reported have been of limited application owing to the quality of the glass produced. For example, some are restricted to use in bulk oscillator devices because the glass cannot be made into optical fibers. Also, many studies have concentrated in detail on a particular property of a chosen doped tellurite, such as energy transfer between dopant ions, modification of the emission spectra by varying the proportion of components of the glass, or generation of a particular wavelength. Such studies are of little use in determining the presence or absence of the full range of physical and optical characteristics required of a glass if it is to be versatile and well-suited to particular applications.
Accordingly, a first aspect of the present invention is directed to a tellurite glass material having a composition of Li2O:TiO2:TeO2, and containing a dopant comprising ions of a rare earth metal. This composition of tellurite glass, referred to herein as LTT glass, has proved to provide a low phonon energy host material for rare earth ions, so that high quantum efficiencies can be achieved. It has proved capable of receiving high levels of dopant without disproportionate increases in the non-radiative recombination rate, so that high levels of optical gain can be achieved. Similarly, the dopant does not appear to have a detrimental effect on the physical properties of the glass. The titanium has been found to make the glass particularly stable, and importantly, its presence appears not to affect the spectroscopy of the dopant ions. Also, the glass shows no crystallisation or devitrification. The combination of these properties makes it highly suitable for making quality optical fibers, as well as other optical structures such as planar waveguides. In particular, the glass is an attractive candidate for the fabrication of waveguides by the dip spin coating technique. The refractive index of the glass can be selected by varying the amount of lithium; this is thought to arise because the lithium substitutes for the heavier tellerium in the glass matrix. This feature is also of benefit in the fabrication of fibers and waveguides, which require glasses of at least two different refractive indices. The above-mentioned advantageous features combine to render the glass particularly well suited for use in fiber amplifiers, because large amounts of gain can be provided in relatively short lengths of high quality, easily fabricated fiber.
According to various embodiment, the composition of the glass may be such that it comprises 5 to 30 mole % of Li2O or 15 to 25 mole % of Li2O; 2.5 to 10 mole % of TiO2 or 4 to 6 mole % of TiO2; and 60 to 92.5 mole % of TeO2 or 70 to 80 mole % of TeO2. Varying the amounts of the various components allows properties of the glass, such as refractive index and the stability, to be altered.
Advantageously, the dopant comprises ions of thulium. Thulium ions have an energy level structure such that they emit light at 1470 nm. This is a desirable telecommunications wavelength because it extends the bandwidths commonly used with existing silica-based fiber systems, including erbium-doped fiber amplifiers. The LTT glass has been found to be ideally suited as a host for thulium ions, because the matrix itself appears to have a negligible effect on the energy level structure of the thulium ions. This means that the spectral emission properties of the doped glass are invariant with different molar compositions. Equally importantly, the relatively low phonon energy of the LTT promotes radiative decay in the thulium.
In one embodiment, the tellurite glass further comprises a co-dopant of ions of holmium. The holmium ions act as acceptor ions which can assist in depopulation of the lower transition level in the thulium ions. This improves the population inversion, and hence improves the available optical gain. Alternatively, at least one of ytterbium, terbium or dysprosium may be used as a co-dopant ions, to provide a similar benefit.
In alternative embodiments, the dopant comprises ions of at least one of erbium, ytterbium, neodymium, praseodymium and holmium. These, and other, rare earth metals can be used as desired, alone or in combination, to achieve various effects. For example, different dopants provide gain at different wavelengths. Alternatively, a dopant may used in combination with a dopant providing gain to suppress unwanted amplified spontaneous emission (ASE), such as neodymium used in the cladding region of a waveguide having a thulium-doped core region to suppress ASE at 800 nm.
The concentration of the dopant may be up to about 30000 parts per million, up to about 10000 parts per million, or up to about 5000 parts per million. The LTT glass can receive high concentrations of dopant if required, but lower levels may be preferred in some cases because they have been found to have very little effect on the lifetime of the upper transition level of the dopant ions.
A second aspect of the present invention is directed to an optical waveguide comprising a core region having a first refractive index and a cladding region having a second refractive index lower than the first refractive index, wherein at least the core region is fabricated from a tellurite glass material having a composition of Li2O:TiO2:TeO2, and containing a dopant comprising ions of a rare earth metal. The optical waveguide may be fabricated as an optical fiber, or alternatively as a planar waveguide structure.
A third aspect of the present invention is directed to an optical fiber amplifier comprising as its amplification medium an optical fiber comprising a core region having a first refractive index and a cladding region having a second refractive index lower than the first refractive index, wherein at least the core region is fabricated from a tellurite glass material having a composition of Li2O:TiO2:TeO2, and containing a dopant comprising ions of a rare earth metal.
A fourth aspect of the present invention is directed to a laser oscillator comprising a gain medium fabricated from a tellurite glass material having a composition of Li2O:TiO2:TeO2, and containing a dopant comprising ions of a rare earth metal. The gain medium may be in the form of an optical fiber, to provide a fiber laser, or alternatively the laser oscillator may be configured as a solid-state bulk laser by using the glass per se as the gain medium.